1. Field of the Invention
The invention relates to a hub for a twin blade rotor formed as a balance bar for a wind engine, a lift rotor or an anti torque rotor of a helicopter.
The invention relates more precisely to the beam serving as support for the two blades associated with the bearing of the balance bar and for the angle of attack and take up bearings of the centrifugal forces of each blade on the beam. It is known that wind engines use the energy produced by the wind for driving an electric generator or for mechanically driving a motive means for example a pump. The essential member of these wind engines is formed by a rotor in which the number of blades may vary depending on the desired uses. The two bladed rotor is lighter and has found preferable applications in the construction of electric generators of different powers. These rotors must be able to rotate at varying speeds and the angle of attack of the blades with respect to the wind must be adjustable so as to operate with maximum efficiency whatever the speed of the wind. A position must also be provided for feathering the blades should the need arise. All these operating constraints and requirements require robust and reliable equipment, more particularly at the level of the blade support as well as of the hub to which they are fastened. The problem is even more accute when the wind engine is of large size for the centrifugal forces and the bending moments on the blades and on the hub in the case of maximum wind, are extremely high.
2. Description of the Prior Art
This type of large sized wind engine, in order to obtain good efficiency while limiting the weight of the rotor, the rotor of two blade type formed as a balance bar provides an interesting solution. It is known that in this type of rotor each blade is generally supported by a hub body mounted through the universal joint on the rotary shaft so that swinging of one blade with respect to the shaft is compensated by reverse swinging of the other blade and that, in strong gusts, the rotor disk thus assumes a natural balanced position, which appreciably reduces the impaction movements of the hub on the support mast.
The lift rotors and rear anti torque rotors of helicopters are also known of the twin blade type formed as a balance bar whose operating principle is substantially the same as that of the rotor of twin blade wind engine formed as a balance bar. In a particular embodiment of such balance bar rotors, each of the two blades is fixed to the end of the same frame or beam whose central part is gripped in a stirrup itself integral with a rotary shaft forming support mast. Generally, these fixing means between blade and hub must transmit and absorb the centrifugal forces due to the rotation and the bending movements resulting from the aerodynamic forces on the blades. Bearings with metal to metal friction which are currently used wear out rapidly and have insufficient resistance for the above mentioned uses. Furthermore, ball or roller bearings are not suitable either, for despite good oil or grease lubrication they quickly deteriorate through "brinellage" because of the high centrifugal force which acts on them and because they do not rotate but oscillate over the same reduced angular sector.
Other known devices for fixing blades to a hub use instead a support capable of receiving radial thrust elastomer bearings comprising truncated cone shaped layers adhering together with alternately metallic and resilient layers which converge in the direction of the blade. It is known that these types of stratified elastomer bearings are characterized by high flexibility and deformability under shearing forces, as well as by low deformation under compression forces and provide good endurance in service and good operating safety because of the slow and visible progression of damage and by suppressing lubrication results in a very high reduction of the surface maintenance costs.